Mr Trump said it was the fact that he believed that Mr Sanders would be denied the nomination which influenced his decision.

“Based on the fact that the Democratic nominating process is totally rigged and Crooked Hillary Clinton and Deborah Wasserman Schultz will not allow Bernie Sanders to win, and now that I am the presumptive Republican nominee, it seems inappropriate that I would debate the second place finisher,” Mr Trump said.

Bernie SandersCredit:
Chris Tilley/Reuters

“Likewise, the networks want to make a killing on these events and are not proving to be too generous to charitable causes, in this case, women’s health issues.

“Therefore, as much as I want to debate Bernie Sanders - and it would be an easy payday - I will wait to debate the first place finisher in the Democratic Party, probably Crooked Hillary Clinton, or whoever it may be.”

Waving US and Mexican flags, more than 1,000 people turned out for anti-trump rallies in the city on the US-Mexico border whose San Ysidro port of entry sees nearly 300,000 people a day cross legally between the countries.

Mr Trump has weathered months of blowback from all ends of the political spectrum for his immigration policy, which calls for the building of a wall along the U.S.-Mexico border and deporting the nearly 11 million undocumented immigrants who reside in the United States.

Critics have said his plan is needlessly cruel and impossible to implement. At Trump's campaign stops, attendees often chant "build the wall."